Presently, the well-known paper currency number recognition device acquires an image by means of a contact image sensor CIS or a CCD image sensor. When a paper currency passes through a pathway of the device, the above-mentioned image sensor collects paper currency images in high speed. An analog to digital (AD) converter converts the paper currency images into image data, and a digital signal processor processes the image data quickly, to determine the position of the paper currency number, and the paper currency number is recognized by applying a recognition algorithm.
Since the number is usually printed on only one side of the paper currency and the direction in which the paper currency passing through the paper currency processing device is in random, generally the paper currency number is acquired by a way of transmission imaging, it's only needed to install one CIS on a paper currency transport pathway. A group of white light LED light sources is installed in front of the CIS, and a passing paper currency is set to be irradiated in a certain irradiation time Photographic imaging is formed by the lights that pass through the paper currency and fall on CIS. No matter the paper currency appears at a positive side or an opposite side of CIS, the paper currency number can be acquired.
However, a layer of dust is accumulated more or less on the surface of most of paper currencies which have been circulated for a period of time at the market, and therefore the energy of the light transmitted to an image sensor in CIS becomes weaker, which results in image blurring and illegible, and a recognition rate of the algorithm is reduced seriously. In the case that the turn-on time of the white light LED is lengthened alone, the energy of the light is strengthened, the old paper currency image becomes clearer and can be recognized, but the outline of the new paper currency image disappears due to the well transmission effect of the new paper currency. Therefore the recognition algorithm can not determine the position of the new paper currency number and thus causes recognition errors. By way of the conventional transmission imaging, it is impossible to form clear images for both the new paper currency and the old paper currency, and a problem for the recognition algorithm is caused, and paper currency number recognition is error prone.